Missing required key on project.conf causes wrong provenance file name on error message

Summary

Steps to reproduce

What is the current bug behavior?

If a required key is missing on project.conf, such as name, BuildStream shows the wrong provenance file name on the error message when trying build the project. See below.

What is the expected correct behavior?

The filename being shown on the error message to point to project.conf instead of projectconfig.yaml.

Relevant logs and/or screenshots

bst build hello.bst

Error loading project: /home/tiagogomes/repos/buildstream/buildstream/data/projectconfig.yaml [line 8 column 0]: Dictionary did not contain expected key 'name'

BuildStream version affected: /milestone %BuildStream_v1.x

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